quinta-feira, 14 de novembro de 2013

Simple physics and climate

No doubt, our climate system is complex and messy. Still, we can
sometimes make some inferences about it based on well-known physical
principles. Indeed, the beauty of physics is that a complex systems can
be reduced into simple terms that can be quantified, and the essential
aspects understood.
A recent paper by Sloan and Wolfendale (2013)
provides an example where they derive a simple conceptual model of how
the greenhouse effect works from first principles. They show the story
behind the expression saying that a doubling in CO2 should
increase the forcing by a factor of log|2|. I have a fondness for such
simple conceptual models (e.g. I’ve made my own attempt posted at arXiv) because they provide a general picture of the essence – of course their precision is limited by their simplicity.
However, the main issue discussed in the paper by Sloan and
Wolfendale was not the greenhouse effect, but rather the question about
galactic cosmic rays and climate. The discussion of the greenhouse
effect was provided as a reference to the cosmic rays.
Even though we have discussed this question several times here at RC,
Sloan and Wolfendale introduce some new information in connection with
radiation, ionisation, and cloud formation. Even after having dug into
all these other aspects, they do not find much evidence for the cosmic
rays plying an important role. Their conclusions fit nicely with my own findings that also recently were published in the journal Environmental Research Letters.
The cosmic ray hypothesis is weakened further by observational evidence from satellites, as shown in another recent paper by Krissansen-Totton and Davies (2013) in Geophysical Research Letters,
which also concludes that the there is no statistically significant
correlations between cosmic rays and global albedo or globally averaged
cloud height. Neither did they find any evidence for any regional or
lagged correlations. It’s nice to see that the Guardian has picked up these findings.
Earlier in October, Almeida et al., 2013 had a paper published in Nature
on results from the CLOUD experiment at CERN. They found that galactic
cosmic rays exert only a small influence on the formation of sulphuric
acid–dimethylamine clusters (the embryonic stage before aerosols may act
as cloud condensation nuclei). The authors also reported that the
experimental results were reproduced by a dynamical model, based on
quantum chemical calculations.
Some may ask why we keep revisiting the question about cosmic rays
and climate, after presenting all the evidence to the contrary.
One reason is that science is never settled, and there are still some
lingering academic communities nourishing the idea that changes in the
sun or cosmic rays play a role. For this reason, a European project was
estaqblished in 2011, COST-action TOSCA
(Towards a more complete assessment of the impact of solar variability
on the Earth’s climate), whose objective is to provide a better
understanding of the “hotly debated role of the Sun in climate change”
(not really in the scientific fora, but more in the general public
discourse). psOldenborgh et al. (2013)
also questioned the hypothesised link between extremely cold winter
conditions in Europe and weak solar activity, but their analysis did not
reproduce such claims.